Charge Carrier Dynamics In Graphene-like Two Dimensional Materials Studied By THz Spectroscopy

The two-dimensional?2D?materials,such as graphene and graphene-like materials,have received tremendous attention in optoelectronic and photonic applications,due to atomic layer thickness,large specific surface area,high mechanical strength and flexibility,etc.So far,the ground state properties of this kind of materials have been studied extensively through some basic characterizations;however,the dynamical properties of the non-equilibrium carriers on the excited states remain elusive.The understanding of the dynamical non-equilibrium carriers is helpful not only to the basic photo-physics property but also to the application of devices based on such material.As the low photon energy of terahertz radiation,the terahertz spectroscopy is a sensitive method to probe the dynamics of the mobile charge carriers near the Fermi levels.In this thesis,we employed time-resolved terahertz spectroscopy to investigate the dynamics of photoinduced mobile charge carriers in graphene-like structure.The main contents of the dissertation include:?1?Graphene oxide?GO?is an attractive option for large scale production of graphene.In addition,the graphene obtained by the reduction of GO has inevitable structural defects,and the vacant lattice sites will significantly restrict its conductivity.It has been demonstrated that thermal annealing in hydrogen is an efficient method to reduce defects and heal the lattice in GO samples.However,it is still not clear that how the defects and/or disordering influence on the photoelectric conversion efficiency and the carrier relaxation pathway in GO.Herein,the time-domain terahertz?THz?spectroscopy is employed to characterize the properties of the multilayer GO films which were annealed in hydrogen at various temperatures.Upon photo excitation,a transient increase of the conductivity was observed for the reduced graphene oxide?RGO?samples.The ultrafast carrier relaxation process can be well assigned to the carrier-carrier scattering and carrier-phonon coupling.Our results demonstrated that the RGO films with fewer defects and better lattice structure is successfully manufactured.In addition,by fitting to the Drude model,several electron transport parameters,such as the carrier scattering time,carrier plasma frequency and photoinduced conductivity,are obtained in our multilayer RGO films.?2?The exciton effect plays a dominated role in describing the optical and optoelectronic properties in two-dimensional?2D?transition metal dichalcogenides?TMDs?.However,the nature of the excitons and free carriers has yet to be understood upon the photo excitation in 2D TMDs.Here we first present a study of the dynamical study of excitons and unbound electron-hole pairs using time-resolved terahertz?THz?spectroscopy?TRTS?in a few-layer WS₂ laminate.Our experimental results demonstrate that the Auger recombination is observed in the relaxation process of the mobile charge carriers rather than that of excitons upon photoexcitation.The transient complex THz photoconductivity spectroscopy of WS₂is well described by Lorentz oscillator model of free carriers monitored by the exciton polarization field.Our results provide a comprehensive understanding of non-equilibrium carrier kinetics?both excitons and free carriers?in WS₂ laminate,and should be applicable to other 2D system.?3?The photoinduced conductivity of MoS₂ on sapphire substrate under different atmosphere was investigated using time-resolved terahertz spectroscopy?TRTS?.The negative photoinduced conductivity was observed at the nitrogen atmosphere and the positive photoinduced conductivity was observed at the air and oxygen atmosphere.We confirm that the oxygen-adsorption can cause the decrease of the defect of S-vacancy,which can form the trions as the charge carrier.Due to the larger effective mass of trion,the photoinduced conductivity decreases.Instead,similar phenomenon was not observed in MoS₂ on quartz substrate.The reason was that the boundary of MoS₂ on quartz substrate was irregular,and the oxygen-adsorption was easier to occur.